A retrospective study was carried out in which all patients operated on simultaneously for hypertrophic cardiomyopathy and coexistent coronary artery disease were included. 28 patients were identified, four undergoing mitral valve replacement (MVR) and coronary bypass grafting (CABG) and 24 undergoing left ventricular- myotomy-myectomy (LVMM) and CABG. Mean age at operation was 59 years (range 42 - 74 yrs). Five patients (18%) died as a result of operation, while 3 died after the immediate postoperative period. Mean period of follow-up for the 20 patients currently alive was 4.8 years (range 4 months to 10.8 yrs). 19 patients experienced substantial long-term symptomatic improvement. Long- term relief of chest pain was noted in 17 of the 20 surviving patients. Significant reduction in basal and provocable gradients were demonstrated in this group of patients after operation (P less than 0.001). 5 of the 24 patients undergoing LVMM incurred iatrogenic ventricular septal defect (VSD). The rate of this complication was higher in this group than that reported in all patients undergoing LVMM. This occurred primarily in patients with septa less than 20 mm as measured by echocardiography. VSD was responsible in large part for the relatively high mortality rate. It is suggested that MVR be considered as a preferable operation to LVMM in patients with coexistent CAD and relatively thin septa.